The present invention relates to a method and an apparatus applicable to a multicolor developing device for detecting the density of an image. More particularly, the present invention is concerned with a method and an apparatus applicable to a multicolor developing device for forming in a non-image region of an image carrier a latent image which is representative of an exclusive pattern for density detection, then developing the latent image by toner of any of multiple colors, and then illuminating the resulting toner image to detect image density in terms of the amount of reflection.
Generally, a multicolor developing device installed in an electrophotographic copier or similar color image recorder is provided with an implementation for detecting image density on the basis of which image forming conditions are controlled. Such an implementation uses, for example, a latent image representative of an exclusive pattern for density detection and formed in a non-image region of a photoconductive drum or like image carrier. The latent image is developed by toner of any of multiple colors to become a toner image. This toner image of particular color is illuminated by light so that image density associated with that color is measured in terms of the intensity of light reflected by the toner image. In this kind of implementation, the image carrier has a face member which is constituted by a light transmitting member while a light absorbing member is located at the back of the image carrier, so that light incident to that part of the image carrier where toner is not deposited is transmitted through the image carrier to be absorbed by the light absorbing member.
Although the spectral reflectivity distribution differs from toner of one color to toner of another color, all the toner of various colors show reflectivety of substantially 100% in the infrared range. In the light of this, the light adapted for detection is often implemented by infrared light to accomplish a good reflectivity characteristic with no regard to the color of toner. A current rend in the imaging art is toward furnishing a full-color developing device which uses cyan toner, magenta toner and yellow toner or a multicolor developing device which uses red toner, blue toner and green toner with a black-and-white reproduction capability which uses black toner independently of the others. With such a type of full-color or a multicolor developing device, it is impossible to achieve high detection sensitivity because the reflectivity of black toner is low (substantially zero) even in the infrared range and, therefore, a sufficient amount of reflection is unattainable despite the use of infrared light for the detection.